The present invention relates generally to predistortion for non-linear devices and, more particularly, to methods and apparatus for providing selective narrowband feedback to an adaptation circuit for a digital predistorter.
The design of radio-frequency power amplifiers for communications applications often involves a trade-off between linearity and efficiency. Power amplifiers are typically most efficient when operated at or near the saturation point. However, the response of the amplifier at or near the point of saturation is non-linear, resulting in both phase and amplitude distortion. The non-linear response of a power amplifier causes out-of-band emissions and reduces the spectral efficiency in a communication system. In most communication systems, constraints are imposed on the non-linearity effects of the transmitted signal both within and outside the operational band.
One way to improve a power amplifier's linearity is to digitally predistort the input to the power amplifier to compensate for the distortion introduced by the power amplifier. In effect, the input signal to the power amplifier is intentionally distorted so that the added distortion, i.e. predistortion, cancels the distortion introduced by the power amplifier. Generally, the predistortion is applied to the signal digitally at baseband frequencies, i.e., before the signal is up-converted to radio frequencies. The appropriate distortion is determined by a non-linear distortion model that is updated based on feedback from the output of the power amplifier. To the extent that the predistorted power amplifier output results in net distortion, the non-linear model can be adjusted based on the feedback to reduce the net distortion at the output.
Non-linear distortion appearing near the operating band of interest can be modeled mathematically as a series of real-valued, odd-order terms of an AM-AM (amplitude modulation-amplitude modulation) response and an AM-PM (amplitude modulation-phase modulation) response which are due to amplitude-distortion and phase-distortion phenomena respectively. These two phenomena can also be described jointly using a complex gain response with complex coefficients for each term. This is referred to herein as the non-linear model. In the frequency domain, the odd-order terms produce distortion spectra with a bandwidth that is proportional to the order. For example, the 3rd-order distortion spectrum, which is due entirely to the 3rd order term, has a spectrum three times the bandwidth of the linear input spectrum. Similarly, the 5th order term has a spectrum five times the bandwidth of the linear input spectrum, and so forth. These spectral components superimpose to form the composite spectrum. Thus, by filtering one can isolate a band of distortion noise such that it is unrelated to the composite linear-term and 3rd-order term; isolate another band of distortion noise such that it is unrelated to the composite linear-term, 3rd-order and 5th-order terms, and so forth.
The magnitude of distortion caused by each term is linked to the non-linear model, and in general diminishes for higher-order terms. This factor has an important implication: for example filtering to select the 5th-order band with linear-term and 3rd-order distortion filtered out contains 5th, 7th, 9th etc, however it is dominated by the 5th-order and hence can be used to estimate the 5th-order component of the non-linear model. Another implication is that very higher order terms become so low that they are insignificant. Significance is usually assessed in context of the constraints on the linearity of the transmitted signal both within and outside the operational band, where the former impair usability of the intended signal and the latter interfere with adjacent signals. The subset components of the non-linear model that are operatively processed are referred to herein as the predistortion model.
In order to properly update the predistortion model, the bandwidth of the feedback path must be large enough as to capture all distortion spectrum considered significant. In addition, the instantaneous dynamic range of the feedback signal must be large enough to represent both the largest magnitude components (linear term) and the lowest magnitude term of significance. In the sampled-domain where digital signal processing (DSP) is performed, a larger number of bits per sample (higher bit resolution) is required to represent a signal with larger instantaneous dynamic range. Total DSP resource loading is proportional to both bandwidth (sampling rate) and resolution (bits per sample). For some applications, the bandwidth and the instantaneous dynamic range of signals demand very high performance in the analog-to-digital-converter (ADC) and other DSP devices within the feedback path. The design criteria of power amplifiers is often driven by three principal opposing considerations: large desired operating bandwidth, stringent in-band linearity requirements associated with high-complexity modulation, and stringent out-of-band emissions regulatory requirements. The bandwidth and resolution requirements for the pre-distortion feedback path required to jointly fulfill these design criteria can be reduced by the present invention and thus can reduce the cost and complexity of predistortion circuits used in connection with power amplifiers.